This invention relates to thin film magnetic recording media and more specifically to magnetic media covered with material which prevents corrosion, improves wear resistance, and reduces the head to media dynamic and static friction coefficients.
Metallic magnetic thin film disks used in memory applications typically comprise a substrate material which is coated with a magnetic alloy film which serves as the recording medium. Typically, the recording medium used in such disks is a cobalt-based alloy such as Co-Ni, Co-Cr, Co-Ni-Cr, Co-Pt or Co-Ni-Pt which is deposited by vacuum sputtering as discussed by J. K. Howard in "Thin Films For Magnetic Recording Technology: A Review", published in Journal of Vacuum Science & Technoloqy, in January, 1986, incorporated herein by reference. Other prior art recording media comprises a Co-P or Co-Ni-P film deposited by chemical plating as discussed by Tu Chen et al. in "Microstructure and Magnetic Properties of Electroless Co-P Thin Films Grown on an Aluminum Base Disk Substrate", published in the Journal of Applied Physics in March, 1978, and Y. Suganuma et al. in "Production Process and High Density Recording Characteristics of Plated Disks", published in IEEE Transactions on Magnetics in November, 1982, also incorporated herein by reference. Several problems are encountered in using unprotected metallic thin film recording media. For example, unprotected metallic thin films tend to corrode, particularly under high humidity conditions. Further, such films have very little resistance to wear caused by frequent contact with the recording head.
To prevent these problems, it is known in the art of recording technology that overcoating thin film magnetic recording media with a hard protective layer such as a carbon or SiO.sub.2 layer improves the wear resistance of the recording media and also provides some corrosion protection to the magnetic film in a low humidity and low temperature environment. Carbon overcoatings for magnetic disks are described by F. K. King in "Datapoint Thin Film Media", published in IEEE Transactions on Magnetics in July, 1981, and Japanese Patent Application No. 58140/77 filed May 18, 1977 by Hinata et al., incorporated herein by reference. It is possible, in principle, to increase the corrosion protection by increasing the thickness of the carbon or SiO.sub.2 overcoat. However, the maximum thickness of the overcoat that is tolerable for high performance disk media is about 2 microinches to permit the read-write head to fly close to the media. Of importance, the electrical performance of the disk is improved as the overcoat is made even thinner. The decrease in overcoat thickness decreases the "effective flying height" of the head on the media (i.e. the gap between the surface of the head and the magnetic layer), thereby improving the signal to noise ratio (S/N), resolution, and overwrite characteristics of the recording media. Unfortunately, if the carbon or SiO.sub.2 overcoat thickness is less than 2 microinches, the overcoat does not provide sufficient corrosion protection for the magnetic media.
To improve the corrosion protection of the magnetic media provided by carbon, it is known in the art to deposit a thin chromium layer between the magnetic thin film media and the overcoat layer. In this multilayer overcoat structure, the chromium layer provides enhanced corrosion resistance while the carbon overcoat provides good wear resistance. However, in order to have effective corrosion resistance as well as good wear resistance provided by the carbon-chromium multilayer structure, the total overcoat thickness must be greater than 2 microinches, which is not desirable for a high performance disk.
As mentioned earlier, the overcoat must not only protect the magnetic film from corrosion, but it must also protect the magnetic film from wear. A further requirement is that the static and dynamic friction coefficients between the read-write head and the overcoat must remain low over a large number of start/stop cycles. (The static friction coefficient is the ratio of lateral force to the normal loading force on the head as the disk starts to rotate. The dynamic friction coefficient is the ratio of the lateral force to the normal loading force after the disk has started to rotate.) If the static friction coefficient between the head and the overcoat material is too high (greater than 1.0), a small motor used in the drive will have difficulty starting rotation of the disk from a stationary position, and if a large motor drives the disk, the motor may cause the head to break off from the head suspension. In addition, if the static and dynamic friction coefficients are too high, mechanical contact between the read-write head and the disk will cause excessive wear in the overcoat and eventually a head crash.
When the disk is rotating in the drive, the head "flies" at a typical distance of about 5 microinches to 15 microinches above the disk. When the drive is turned off, the head comes into physical contact with disk. Since the drive is likely to be repeatedly turned on and off during its lifetime, the overcoat must protect the magnetic film from wear, and at the same time, the static and dynamic friction coefficients between the head and the overcoat must remain low after repeated start/stop cycles. It has been demonstrated that even though hard carbon and SiO.sub.2 overcoats resist wear well, static and dynamic friction coefficients increase dramatically after repeated start/stop cycles.
Because of the above-described mechanical and corrosion problems, it would be desirable to coat a magnetic disk with an overcoat material which would improve corrosion protection of the magnetic film without being excessively thick and at the same time exhibit good wear resistance and consistently low static and dynamic friction coefficients.